Threaded joint for steel pipes containing a sealing device located at the level of the threading

ABSTRACT

A threaded joint for steel pipes useable in particular for the petroleum industry is provided. The joint is formed by threaded male and female elements wherein one part of the thread of a first element is strongly applied against the thread of the other element. One of the elements contains a groove having a length of at least one thread pitch and a depth greater than the height of the threads. A sealing device is provided in the groove consisting of a synthetic lining whose volume and shape essentially correspond to the volume and shape of the play that would exist between the threads of the elements.

BACKGROUND

The present invention concerns a thread joint for steel pipes, usable inparticular in the petroleum industry, which comprises a sealing devicelocated at the level of the threadings.

It is known that joints for pipes used in the petroleum industry mustpresent a certain number of characteristics that are difficult toreconcile and which consist in particular of the fact that the jointwhile being easy to machine and not very sensitive to damage, permitsensurance of perfect sealing with respect to very strong pressures thatare exerted either on the inside or outside of the joint, even under thehypothesis where the joint is subjected to bending or substantialtension or axial compression.

It is also necessary that the elements of the joint be interchangeablewithout having an unfavorable effect on the quality of sealing obtainedduring new screwing of the joint with other elements. The threadedjoints now used in the petroleum industry mostly comprise male andfemale threadings that are most often conical (but which can also becylindrical) and which ensure mechanical maintenance of the two elementsof the pipe, one within other, essentially supporting the axial tensionsexerted on the joint.

In most cases the known joints also comprise so-called screw stops whosepurpose is to immobilize the male element relative to the female elementas soon as the prescribed tightening torque has been reached.

Finally, the joints of this type also contain at least one pair ofsealing surfaces that are placed so that during tightening of the jointthese sealing surfaces come in contact with each by being applied toeach other by a sufficient pressure to obtain sealing.

In most cases this is achieved by contact of the metal-metal type, butin certain cases the sealing surfaces can be associated with joints madeof plastic, such as PTFE rings which by deformation ensure or contributeto sealing of the joint.

It is known in particular that such annular joints, for example, made ofPTFE, can be placed in a groove made on the female element, or at thelevel of the sealing surfaces, or even at the level of the threadings sothat during tightening of the joint the male element causes plasticdeformation of the joint which loses its initial shape and adapts itselfto the shape of the male element.

Such plastic joints, however, present the drawback of being difficult toemplace and in practice require changing each time the joint isdisassembled and then reassembled.

Such joints consisting of plastic rings that are simply positioned in agroove made in the female element must in effect be very stronglydeformed in order for the male element to inlay its threadings.

In practice this causes destruction of the plastic joint, which is thusunusable for later installation of joint elements. Moreover, thesubstantial stresses imposed on the plastic joint during thisdeformation often cause displacement of the plastic ring which is thusextracted from the groove and no longer functions in its role.

It is also known, in particular for achieving sealing of screwed jointsintended to supply buildings with water or central heat, that there is atechnique consisting of covering the threading of one of the elements bymeans of a thermoplastic material that is deformed during screwing toensure tightness.

These sealing devices are intended to replace the use of flax or PTFEstrip which are conventionally wound onto the threading of the maleelement before screwing into the female element. These sealing deviceseek to preposition a thermoplastic material at the level of thethreading of these elements.

In this case as well it is generally not possible to lossen the joint,to retighten it by interchanging the elements, preserving the quality oftightness required.

The present invention seeks to impart to joints for steel pipes,intended particularly for the petroleum industry, satisfactory sealingat the level of the threadings, the sealing being achieved by a plasticelement whichis an integral part of one of the elements of the joint,which can be reused reliably during subsequent tightenings of thiselement.

It is known that in order to permit its original function, which is tomechanically connect the two elements of the pipes, each thread of themale element must be applied to a corresponding thread of the femaleelement necessarily in the axial direction and most often (in the caseof truncated threadings) in the radial direction as well.

It follows that one can break down the contour of the threads of onejoint in the clamped condition, on the one hand, into surfaces that arestrongly applied to each other during screwing of the joint, and on theother hand, into surfaces of the male element and female element thatare opposite each other at a certain distance from each and betweenwhich the play existing in the threading is located.

It follows from the existence of this part of the contour of thethreadings where the surfaces ofthe male and female elements areseparated from each other, that to date it has not been possible inpractice to achieve satisfactory sealing at the level of the threadingsof this type of joint.

In effect, the existence of this play between the corresponding surfacesof the male threading and female threading creates a leakage path withessentially helicoidal shape along the threadings, which opposes anysealing at this level.

The present invention is based on the achievement of sealing which islocated along this helicoidal leakage path which can thus be blockedeffectively and repeatedly.

SUMMARY OF THE INVENTION

The object of the present invention is a threaded joint for steel pipesused in particular for the petroleum industry, of the type in which themale element and the female element are connected by threadings, whereasone part of the surface of the contour of each thread is stronglyapplied in the tightened condition to the threading of the otherelement, characterized by the fact that at least one of the elements ofthe joint contains over an axial distance corresponding to the length ofat least one thread pitch, at least one groove whose depth is greaterthan the height of the threads, said groove or grooves being equippedwith a lining made of a synthetic material whose volume and shapecorrespond at least to the volume and shape of the plat that would existin the tightened condition between this element of the joint and theother element of the joint whose dimensions will be at the limit ofmachining tolerances that permits the greatest possible play of thejoint in the installed condition.

One understands that under these conditions the joint according to theinvention consists of a first joint element that is provided with atleast one groove filled with a synthetic lining made to conform beforescrewing and whose external shape is strictly complementary to the shapeof the contour of the threads of the other element after screwing(accounting for the compressions that can occur during tightening of thejoint), and a second conventional element that is totally metallic oralso contains at least one groove provided with a lining like the firstelement.

One also understands that in such a joint the helicoidal leakage pathwhose existence was mentioned above is interrupted in sealed fashin atthe level of the groove or grooves, because of the fact that when thejoint is tightened, the synthetic lining is elastically supported on theentire periphery of the threading of the element.

Because of the fact that the mass of synthetic material in the groove orgrooves has not been shaped during tightening to adapt to the shape ofthe threading of the other element, this synthetic material simplyundergoes slight compression, but is not subjected to deterioration anddoes not have to be replaced with each tightening, as was previously thecase when one positioned rings made of a synthetic material, for examplePTFE, in the grooves machined in the female element.

Moreover, this synthetic material, by adhering strongly to the walls ofthe groove or grooves, does not risk being accidentally removed.

According to the invention, the groove or grooves made in at least oneof the elements at the level of the threading advantageously presents adovetail so as to avoid a situation where the lining made of a syntheticmaterial will have a tendency to be detached from the groove where it ishoused under the influence of stresses communicated during tightening ofthe other element.

In a preferred version, comprising a single groove that receives thesynthetic lining, this is achieved on the female element in the vicinityof the end of the threading which is located next to the end of the maleelement.

However, such a groove can also be made at another point of thethreading of the female element or even at any point of the threading ofthe male element, without departing from the scope of the presentinvention.

The synthetic lining that is contained in the groove or grooves made inat least one of the elements can be obtained by various processes.

Accoring to a first process of the invention one screws onto or into theelement containing the groove or grooves a mandrel containing athreading whose dimensions correspond to the minimal tolerance limits ofthe joint element that must be tightened in or on the element thatcontains a groove or grooves, or whose dimensions are slightly less thanthese minimal limits, when one desires to systematically obtaincompression of the plastic linings, and by means of a number of channelsarranged in the mandrel one injects at the level of the groove orgrooves the synthetic material which, adhering to the surface of thegroove after polymerization, thus produces a synthetic lining accordingto the invention, which is integral with the considered joint element.

As a variant, one can use a mandrel devoid of channels which is screwedinto or onto the element containing the groove or grooves after havingapplied in it a sufficient mass of synthetic material.

According to another process of the invention one applies in the grooveor grooves made in one of the joint elements a sufficient mass ofsynthetic material in which one causes polymerization, whereupon thissynthetic material is machined to impart to it a shape which,considering the compression one wishes to achieve, exactly correspondsto the contour of the threading of the other element in which thedimensions correspond to the minimal manufacturing tolerances for thisother element.

According to this invention, it is advantageous to use as syntheticmaterial urethane elastomers such as "Polathane XPE or RIM 160" with amicrocellular structure, polycarbonates, polyolefins or polyamides.

BRIEF DESCRIPTION OF THE DRAWINGS

For purposes of better understanding of the invention several versionsshown in the accompanying drawings will now be described asillustrations that are in no way limiting. In the accompanying drawing:

FIG. 1 depicts a cross section of a joint whose female element isequipped with a groove and synthetic lining according to the invention.

FIG. 2 is a cross section on a larger scale along II of FIG. 1 showingthe play that exists at the level of the threadings outside of syntheticlinings according to the invention.

FIG. 3 is a cross section on a larger scale of part III of FIG. 1.

FIG. 4 is a schematic cross section showing a version of the syntheticlining according to the invention.

DETAILED DESCRIPTION

In FIG. 1 a half-cross section of a joint according to the invention isshown.

This joint consists of a female element, or sleeve 1 which contains aninternal conical threading 2 in which the male element 3 is engaged by acorresponding external conical threading 4.

A stop 5 arranged within the female element serves to support the end 6of the male element to limit tightening, whereas a pair of surfaces 7and 8 arranged respectively on the female element and male elementensure tightness of the joint of the metal-metal type.

One sees in FIG. 2 how thread 9 of male element 3 is engaged in a recess10 of female element 1. Considering the existence of the screw stop 5and the conicity of the threadings, the side 11 of thread 9 is stronglyapplied axially against the side 12 of recess 10. Moreover, the bottomof thread 13 of male element 3 is strongly applied radially against thetop of thread 14 of the female element.

In the version shown it is the tops of the female threadings that aresupported against the hollows of the male threadings, but this could beopposite, if during machining one made the height of the threads 9greater than that of the recesses 10.

Under these conditions, if one considers the complete periphery of onethread of the male element, one finds that on the bottoms of thread 13and the sides of thread 11, the female element is applied stronglyagainst the male element which ensure tightness in this zone. On theother hand, at the level of the tops 15 and the back side 16 of thread 9of the male element there is necessarily a play between the femaleelement and the male element. This play shown at 17 in FIG. 2 means thatit hitherto has not been possible to obtain satisfactory sealing at thelevel of the threadings.

One understands in effect that this play, which is shown in FIG. 2 atthe level of one thread, continues helicoidally along the threadingbetween the male element and the female element.

The synthetic lining 18d that is seen in FIG. 1 has the purpose ofinterrupting this leakage path by completely obstructing this play atthe level of at least one thread pitch.

FIG. 3 shows on a larger scale part III of FIG. 1.

One sees in FIG. 3 the groove 18 provided with a synthetic lining 18d.One notes that the bottom of groove 18a which is essentially parallel tothe threading, according to the invention is beyond the hollows of thethreads of the element that contains the groove. Preferably, the depthof the groove is essentially equal to about twice the height of thethreads.

According to a preferred of the invention, the lateral edges 18b and 18cof the groove are inclined to impart to the latter a section in theshape of a dovetail, surfaces 18a, 18b and 18c being connected to eachother by flaring.

One sees in FIG. 3 that according to the invention the synthetic liningcontained in the groove after tightening occupies the entire free spacearound the threadings of element 3.

In particular, one sees that the synthetic material is appliedcontinuously over the successive parts 11, 15, 16, 13, etc. of thethreading of element 3. As mentioned before the width of the groove andthe synthetic lining is greater than one thread pitch in order to becertain that the helicoidal leakage channel is obstructed.

According to the invention the inside periphery of the synthetic liningin groove 18 must be such that it leaves no play between itself and thethreading of the other element, even when the threading of this otherelement is made with minimal dimensions which account for machiningtolerances.

According to this invention, it is preferable that, whatever the exactdimensions of the element that is tightened in the element containingthe groove and the synthetic lining (these variations in dimensions canbe due to machining tolerances), a slight compression is exerted betweenthe synthetic lining and the threading of the other element.

In this fashion one obtains tightness at the level of the threadingswhich is of good quality and remains reliable, even after numeroustightenings and loosenings of the joint with interchanging of elements.

This sealing according to the invention can naturally be used alone orin succession with other sealings, such as metal-metal sealing, which isobtained on surfaces 7 and 8 of the joint shown in FIG. 1.

FIG. 4 shows a device that permits use of processes that produce asynthetic lining according to the invention.

This FIG. 4 shows the same female element 1 as in FIG. 1.

A mandrel 19 which is screwed into the female element 1 contains anexternal truncated threading whose shape corresponds to the minimalpossible dimensions for the other metal element, considering themachining tolerances of threading. In the case where on wishes tosystematically obtain a slight compression of the synthetic material ingroove 18, the dimensions of mandrel 19 are slightly less than theminimal dimensions permitted by the manufacturing tolerances of thejoint threading.

The mandrel 19 contains a series of longitudinal channels 20 perforatedon its periphery, which are connected by radial channels 21 to the spaceof groove 18.

To produce the synthetic lining, after having applied an antiadhesiveproduct to the corresponding surface of the threadings of mandrel 19,one screws the latter into the female element 1 and then injects thesynthetic material through the channels 20 to line the space of groove18.

After polymerization of the synthetic material one loosens the mandreland obtains the synthetic lining according to the invention, which willimpart perfect tightness and repeated tightness at the level of thethreadings, when any male element is screwed into female element 1.

According to another manufacturing process one applies in groove 18 asufficient amount of synthetic material which is polymerized whereuponafter the synthetic material is hard enough, one carries out mechanicalmachining to impart the profile of the threading of the male elementwith dimensions corresponding to that which was discussed previously.

The plastic material used is a compound of urethane such as is knownunder the name "Polathane XPE."

It is understood that the versions that have been described above arenot limiting and any desirable modifications could be made withoutdeparting from the scope of the invention.

In particular, it is obvious that, although in the versions described itis the female element that is provided with the groove containing thesynthetic material, it is also possible to make a groove containingsynthetic material on the male element.

It also goes without saying that the groove can be positioned atdifferent sites of the threading and that one could even provide togrooves lined with a synthetic material according to the invention.

Finally, it is obvious that the sealing device according to theinvention can be used alone or associated with other sealing devices,for example of the metal-metal type, and that the joints to which theinvention applies can contain screw stops or not.

Moreover, the invention can be applied to joints whose threadings aretruncated or to joints whose threadings are cylindrical.

Finally, it would be possible to make a groove on the female element anda groove on the male element, these two grooves corresponding so thatsealing is achieved at the level of the two synthetic linings.

We claim:
 1. A threaded joint for steep pipes usable in particular inthe oil industry comprising a male element and a female elementconnected by threadings on each element, one part of the surface of thethreading of one element being strongly applied in the made up conditiononto the threadings of the other element, at least one of said elementsof the joint comprising over an axial distance corresponding to at leastone thread pitch, at least one groove having a depth greater than theheight of the threads, said groove containing a sealing, syntheticlining adhering only to said element, the volume and shape of whichcorresponds substantially to the volume and shape of the clearance thatwould exist in the made up condition of the joint between the elementcomprising the groove and the other element of the joint, the dimensionsof which would be at the limit of the machining tolerances allowing thegreatest possible clearance between the two elements of the joint infront of the grooves in the made up condition, so that the syntheticlining will not be subjected to any substantial permanent deformationduring successive tightenings, said lining being applied prior to makeup.
 2. A process for producing a lining in a joint according to claim 1,said process comprising:screwing onto the male element of the jointcontaining the groove, a mandrel comprising a threading the dimensionsof which correspond substantially to the limit of tolerances of thefemale joint element that must be screwed onto the element for obtainingthe largest possible clearance between the two elements, and injectingthrough channels disposed in the mandrel at the level of the groove thesynthetic material which adheres to the inside of the groove afterpolymerization for constituting the synthetic lining.
 3. A process forproducing a joint according to claim 1 comprising:applying in the groovein one of said elements a sufficient mass of crude synthetic material tocompletely fill the groove and area corresponding to the threads;inducing polymerization of the synthetic material; and machining thepolymerized synthetic material in order to impart to it a shape whichconsidering the compression desired to be achieved in the syntheticmaterial, corresponds to the shape of the threading of the other elementwhose dimensions correspond to the minimal manufacturing tolerances. 4.A process for producing a lining in a joint according to claim 1, saidprocess comprising:screwing into the female element of the jointcontaining the groove, a mandrel comprising a threading the dimensionsof which correspond substantially to the limit of tolerances of the malejoint element that must be screwed into the female element for obtainingthe largest possible clearance between the two elements, and injectingthrough channels disposed in the mandrel at the level of the groove thesynthetic material which adheres to the inside of the groove afterpolyermization for constituting the synthetic lining.
 5. A threadedjoint according to claim 1, in which the dimensions of the syntheticlining slightly exceed the dimensions corresponding to the limit ofmachining tolerances which permits the greatest possible play of thejoint in the installed condition.
 6. A threaded joint according to anyof claims 1 or 5, in which the groove has a cross section in the shapeof a dovetail.
 7. A threaded joint according to any of claims 1 or 5, inwhich the synthetic lining adheres to the surface of the correspondinggroove.
 8. A joint according to any of claims 1 or 5 comprising a singlegroove containing a lining located on the threading of the femaleelement in the vicinity of the end of the male element.
 9. A jointaccording to any of claims 1 or 5 in which the synthetic lining is aurethane elastomer such as polathane XPE or RIM 160 with microcellularstructure, polycarbonates, polyolefins or polyamides.